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THE ADVANTAGE OF ALL-AERODERIVATIVE OR ALL-ELECTRIC DRIVEN TECHNOLOGY
Large frame gas turbine driven LNG modules can be inefficient, inflexible and wasteful for new LNG projects. Future LNG plants would be best served by all-aeroderivative driven or allelectric driven technology. These concepts can offer 10 to 20% more efficiency compared to heavy frame units.
The provision of LNG refrigeration drive power has traditionally required the use of a tandemdriveofheavyframeindustrial- typegas turbines and electric motors. The electric motor is termed a “starter helper motor” (SHM) as it starts the gas turbine and compressor.
Once up to full speed, the SHMsupplies active torque to increase output power especially at elevated ambient temperatures. In cold conditions, the SHMcan act as a generator to export excess power. Significant torque is required during startup since the compressor starts loaded with positive pressure inside (to avoid wasteful and emissionintensive de-pressurization).
A variable speed drive (VSD) is used to provide this feature. A load-commutated inverter (LCI) drive with brushless synchronous motor is usually selected for this duty. Alternatively VSI (voltage source inverter) technology with an induction motor may be used.
SHM drives typically have less than 50 MWat 3,000 or 3,600 rpm. SHM drives should be incorporated into the plant’s electrical scheme. SHMusually features a double-ended shaft inverter-fed electric motor between the GT and compressor. Side effectswhen usingLCI drives such as harmonic interactions should be studied andmitigated.
There also has been interest shown in eliminating gas turbines for LNG and deploying electric drive motors. The advantages cited include safety, efficiency and availability. However, the drawback is high capital expenditure for total plant including the power generation unit.
A VSD electricmotor, on the other hand, is usually cheaper than a gas turbinewith the same rating.Alternatively, aeroderivative turbines could be used in LNG as they offer more efficiency and flexibility compared to heavy frame turbines.
All-electric and all-aeroderivative designs present challenges.Maintaining an early exchange of information between operator, contractors and vendors can reduce cost and risk by reviewing available designs and options.
Provision of lowtemperature to the LNG processes, for instance, requires refrigeration cycles to liquefy the gas streams.A refrigerant compressor requires considerable amounts ofmechanical power to increase the pressure and condensing temperature of refrigerant vapors.Thismechanical energy is provided by gas turbines or electric motors, which act as compressor drivers.
Proper methodology should be applied to an LNG plant to identify optimal systems at various economic scenarios.Additionally, a systematic methodology for the integrated design of refrigeration and power systems should be addressed.
Restricting the problem only to the minimization of power consumption may seem to reduce costs since fuel and energy accounts for a large portion of lifecycle costs. However, the resulting LNG plant could end up with a poor driver, high initial costs or overly complex LNG/power plant systems. (Note that dissimilar types of gas turbines are not usually acceptable since different maintenance regimes and more spare parts have to be warehoused.)
The availability and reliability of electric motors and SHM applications are only as good as the reliability of their power supply system (the power plant). Apart from harmonic interactions (both harmonic voltage distortion in the supply system and harmonic torques in the motor-compressor string), there are subtle effects that can be problematic if not addressed properly.
Low frequency influences, for example, can influence plant stability. Possible subsynchronous resonances leading to mechanical failure have also been observed. Accurate harmonic and inter-harmonic studies require that consideration is paid to details such as generation train inertia, automatic voltage regulator (AVR) and turbinegovernor response data.
The stability of the electrical power system should be proven even under disrupted process conditions. Operators should be satisfied with dynamic simulations demonstrating the continuity of the liquefaction process under certain disruptive conditions like the loss of a turbo-generator. The remaining healthy generation-sets should be able to assume the load while keeping compressors within their operating envelop.
Special EnergyNetworkMonitoring and Control (ENMC) systems assist in coordinating the individual automation subsystems of an all-electric refrigeration plant and in reestablishing the power balance after the electrical system is disturbed. Dynamic simulations can predict such scenarios and suggest design modifications or mitigation measures to avoid uncertainties and possible risks.
Since all-electric LNG plants usually include a steam system in the combined cycle arrangement, the response of the water-steam cycle elements is critical. If process steam is extracted from the power plant, or steamgenerated and exported to the power plant, the continuous supply of steam should also be included in any simulation.
Vendors should provide actual responsetime constants based on database simulations rather than the typical values from previous projects. RAM (reliability, availability and maintainability) studies can be performed by independent consultants to verify key assumptions.
Unlike fixed-speedmotors, the operation of a VSD-controlled motor usually shows a soft, smooth starting behavior without surges. Generally, the installation of VSD at a plant does not raise major concerns in power plant design. One issue that can cause problems is the inrush current due to the magnetization of the VSD transformer. When this is foreseen, the usual pre-magnetization system through the secondary of the transformer could be implemented aswell as a dedicated requirement concerning the induction value of the equipment.
Another issue relates to load-step. In case of a shutdown of the VSD system, the disturbance can cause instability. In plants where theVSD systemload is a significant percentage of overall power demand, the power plant supplier should verify that it could operate at all known operating and upset conditions. This is particularly relevant for the pressurization sequence of compressors, where the MW/second ramp-up requirement should be matched by plant step-up capabilities.
In addition, logic is required to define load shedding to prevent a plant-wide shutdown in case of losing amajor load or a generator. Further, watch out for inter-harmonic effects on turbine-generator trains.VSD systems can also continuously produce small torque oscillations on the shaft line. Their effects should be analyzed, along with other torsional excitations to prevent mechanical resonance exceeding permissible values.
AminAlmasi is a registered professional engineer in Australia and Queensland (M.Sc. and B.Sc. in mechanical engineering). He is a consultant specializing in rotating equipment, condition monitoring and reliability.